Small watercraft (e.g., PWC) are in some cases provided with a spray strip portion at a front portion of a hull or a bow. The spray strip portion is also called a chine or a spray deflector formed to improve sea kindliness. The spray strip portion is typically a protruding portion formed by curving and protruding outward a part of the hull and by extending it in a longitudinal direction of the hull. One to four spray strip portions are provided on both sides of the hull at different vertical positions.
The spray strip portion is manufactured in such a manner that FRP sheets impregnated with thermosetting resin or the like are stacked to conform in shape to a mold (female mold) of the hull having grooves corresponding to the spray strip portions and are pressed from downward to upward using a roller or the like every time an FRP sheet is stacked, to avoid uneven heating of the spray strip portion in a thermal hardening process.
In this process, the roller falls into the groove portion of the spray strip portion and reduces the thickness of a bottom region of the groove portion, whereas a region immediately above the thinned region is made to have a larger thickness. This results in a hull cross-sectional shape in which stress concentration is likely to occur on the groove portion. Such stress concentration tends to occur particularly on the spray strip portion located higher whenever a front bottom portion of the hull hits on the water surface while skipping on the water surface, thereby decreasing stiffness of the hull.
In addition, as described above, non-uniform thickness may cause the spray strip portion to be unevenly heated and may adversely affect a quality of a product if it is directly thermally hardened.